1. Field of the Invention
The present invention relates to an aqueous pigment dispersion that shows excellent dispersibility and storage stability, which when used as an inkjet ink, offers superior jetability (discharge characteristics), water resistance and adhesion. Furthermore, the present invention also relates to an aqueous inkjet ink that shows excellent storage stability, jetability, water resistance, and adhesion. In addition, the present invention also relates to a process for producing an aqueous pigment dispersion for use as an inkjet ink that shows excellent dispersibility and storage stability, as well as superior jetability, water resistance and adhesion.
2. Description of the Related Art
Currently, solutions of water-soluble dyes such as acid dyes, direct dyes and basic dyes in a glycol based solvent and water are widely used as inkjet inks (Japanese Laid-open publication (kokai) No. Sho 53-61412, Japanese Laid-open publication (kokai) No. Sho 54-89811, and Japanese Laid-open publication (kokai) No. Sho 55-65269). In order to ensure good ink stability, water soluble dyes that show good solubility in water are the most commonly used dyes. However, as a result of this solubility, inkjet inks typically show poor water resistance, and if water is spilt on a printed image, the dyes within the printed areas tend to bleed very easily.
In order to try and alleviate this type of poor water resistance, investigations have been conducted into changing the dye structure, and preparing strongly basic inks (Japanese Laid-open publication (kokai) No. Sho 56-57862). Other research has been conducted into improving the water resistance by effectively utilizing the reaction between the recording paper and the ink (Japanese Laid-open publication (kokai) No. Sho 50-49004, Japanese Laid-open publication (kokai) No. Sho 57-36692, Japanese Laid-open publication (kokai) No. Sho 59-20696, Japanese Laid-open publication (kokai) No. Sho 59-146889). These methods produce remarkable effects for special recording papers, but lack flexibility, as they restrict the types of recording media that can be used, and when media other than the special recording paper are used, a recorded image of satisfactory water resistance can often not be produced using inks that use water soluble dyes.
Furthermore, although water resistant inks produced either by dispersing or dissolving an oil soluble dye in a high boiling point solvent, or by dissolving an oil soluble dye in a volatile solvent do exist, such inks are environmentally undesirable due to problems associated with solvent odors and solvent discharge. In addition, in cases of large volume print recording, or in other cases depending on the location of the printing device, the solvent may need to be recovered, which is difficult. As a result of the above issues, the development of inkjet inks comprising a pigment dispersed in a water-based medium is being actively pursued as a way of improving the water resistance of the recorded product.
However, pigments differ from dyes in that they are insoluble in the recording media, and dispersing fine particles of pigments, and maintaining such dispersions in a stable state, has proven extremely difficult. In addition, as the resolution of printers has increased, leading to finer nozzle diameters, the particle size of colorants within inkjet inks has needed to be reduced even further. This additional reduction in particle size has made it even more difficult to maintain inks in stable dispersions. Further improvements are needed in order to harness the inherent advantages that pigments offer over dyes in terms of water resistance and light fastness, while seeking performance at least equal to, and preferably superior to dyes in terms of other sought-after characteristics such as ink discharge properties, long term storage stability, adhesion to recording media such as paper, image color, and bleeding.